So when i give "sudo mount_ntfs /dev/sd0i /home/sepuku/usb" why do i get an "invalid argument" error?

You have not provided which version of OpenBSD you are running, however, you should familiarize yourself with the information found in Section 14.17 of the FAQ. NTFS support is not available by default in the kernel, so you will need to compile a custom kernel per Section 5.7. Note that support of NTFS is limited to read-only. Writing to NTFS has always been problematic (& not recommended), & writing in the upcoming 5.0 release, is disabled altogether.